Mannyo - sounds like you've had a lot of hassle there
For all those looking to replaces struts I bought some for my Allroad from Germany new for about £140 each. Company was called aerosus or something along those lines.
I never had any problems with it at all. I think buying a used strut is a gamble as you never know whether your used replacement is any better.
Mannyo - I had a replacement strut fitted on my D3 and it was calibrated on the flat. Not sure how a slope would affect things but why risk it?
Hope you get your car sorted. They are good cars the D3's but replacing costly items is a major headache and is where a lot of owners sell up.